Fix reproducibility of provider builds (apache#43557)

Four hours ago Flit released a new version (3.10.0) that bumped
versio of metadata used to generate providers. That revealed a bug
in the reproducibility setup for our providers. Previously the
providers had flit>=3.2,<4 as build dependency - but that caused
newer flit version to be used during the build and - since flit
bumped the metadata-version produced, it caused the packages
generated to be binary non-reproducible.

This PR fixes it by "pinning" flit version (all build dependencies
should generally be pinned to provide reproducibility)

dev/breeze/src/airflow_breeze/templates/pyproject_TEMPLATE.toml.jinja2

@@ -41,7 +41,7 @@
 # `pyproject_TEMPLATE.toml.jinja2` IN the `dev/breeze/src/airflow_breeze/templates` DIRECTORY
 #
 [build-system]
-requires = ["flit_core >=3.2,<4"]
+requires = ["flit_core==3.10.0"]
 build-backend = "flit_core.buildapi"
 
 [project]